The present invention relates to silica-coated carbon products and methods for making silica-coated carbon products. Preferably, silica is coated onto carbon products, such as carbon black without forming free-standing silica particles. The present invention also provides a means for removing metallic cations during the manufacture of silica-coated carbon products.
Carbon blacks are widely used as pigments, fillers, and reinforcing agents in the compounding and preparation of elastomeric compounds used in the manufacture of tires. They are generally produced in a furnace-type reactor by pyrolyzing a hydrocarbon feedstock with hot combustion gases to produce combustion products containing particulate carbon black. Carbon black exists in the form of aggregates. The aggregates, in turn, are formed of carbon black particles. Carbon blacks are generally characterized on the basis of analytical properties, including, but not limited to, particle size and specific surface area, aggregate size, shape, and distribution, and chemical and physical properties of the surface. The properties of carbon blacks are analytically determined by tests known to those skilled in the art. For example, nitrogen adsorption surface area (measured by ASTM test procedure D3037xe2x80x94Method A) is a measure of specific surface area. Dibutylphthalate absorption of the crushed (CDBP) and uncrushed (DBP) carbon black (measured by ASTM test procedures D3493-86 and D2414-93, respectively), relate to the aggregate structure. The properties of a given carbon black depend upon the conditions of manufacture and may be modified, e.g., by altering temperature, pressure, feedstock, residence time, quench temperature, throughput, and other parameters.
It is generally desirable in the production of tires to employ carbon black containing compounds when constructing the tread and other portions of the tire. For example, a suitable tread compound will employ an elastomer compounded to provide high abrasion resistance and good hysteresis balance at different temperatures. A tire having high abrasion resistance is desirable because abrasion resistance is proportional to tire life. The physical properties of the carbon black can directly influence the abrasion resistance and hysteresis of the tread compound. Generally, a carbon black with a high surface area and small particle size will impart a high abrasion resistance and hysteresis to the tread compound. Carbon black loading also affects the abrasion resistance of the elastomeric compounds. Abrasion resistance increases with increased loading, at least to an optimum point, beyond which abrasion resistance actually decreases.
The hysteresis of an elastomeric compound relates to the energy dissipated under cyclic deformation. In other words, the hysteresis of an elastomeric composition relates to the difference between the energy applied to deform the elastomer and the energy released when it recovers to its initial undeformed state. Hysteresis is characterized by a loss tangent, tan xcex4, which is a ratio of the loss modulus to the storage modulus (that is, the viscous modulus to the elastic modulus). Tires made with a tire-tread compound having a lower hysteresis measured at higher temperatures, such as 40xc2x0 C. or higher, will have reduced rolling resistance, which in turn, results in reduced fuel consumption by the vehicle using the tire. At the same time, a tire tread with a higher hysteresis value measured at low temperature, such as 0xc2x0 C. or lower, will result in a tire with high wet traction and skid resistance which will increase driving safety. Thus, a tire tread compound demonstrating low hysteresis at high temperatures and high hysteresis at low temperatures can be said to have a good hysteresis balance.
Silica is also used as a reinforcing agent (or filler) for elastomers. However, using silica alone as a reinforcing agent for elastomer leads to poor performance compared to the results obtained with carbon black alone as the reinforcing agent. It is theorized that strong filler-filler interaction and poor filler-elastomer interaction account for the poor performance of silica. The silica-elastomer interaction can be improved by chemically bonding the two with a chemical coupling agent such as bis (3-triethoxysilylpropyl) tetrasulfane (TESPT), commercially available as A-1210 from Witco, USA and Si-69 from Degussa AG, Germany. Coupling agents such as TESPT create a chemical linkage between the elastomer and the silica, thereby coupling the silica to the elastomer.
When the silica is chemically coupled to the elastomer, certain performance characteristics of the resulting elastomeric composition are enhanced. When incorporated into vehicle tires, such elastomeric compounds provide improved hysteresis balance. However, elastomer compounds containing silica as the primary reinforcing agent exhibit low thermal conductivity, high electrical resistivity, high density, and poor processibility.
When carbon black alone is used as a reinforcing agent in elastomeric compositions, it does not chemically couple to the elastomer but the carbon black surface provides many sites for interacting with the elastomer. While the use of a coupling agent with carbon black might provide some improvement in performance to an elastomeric composition, the improvement is not comparable to that obtained when using a coupling agent with silica.
One method of improving the properties of carbon black so that its properties can be comparable to those of silica is to coat the carbon black with silica. One method is described in Japanese Unexamined Patent Publication (Kokai) No. 63-63755. The carbon black is dispersed in water, followed by adjusting the pH to 10 to 11, and maintaining the temperature to at least 70xc2x0 C. During that time, a diluted sodium silicate solution is added. Then by reducing the pH to 6.5 to 7, the sodium silicate is made to hydrolyze, causing amorphous silica to adhere to or deposit on the surface of the particles of carbon black. The slurry is then filtered and the coated carbon black particles are washed in an attempt to remove dissolved sodium. The silica-coated blacks are then dried. U.K. Patent No. 972,626 and U.S. Pat. No. 5,679,728 state that such silica-coated carbon blacks can be used in rubber formulations.
A disadvantage of such a method for making a silica coated carbon black is the likelihood of forming free-standing particles of silica. Sodium silicate solutions contain both silicic acid and silica particles, mostly 2-3 nm in diameter, but some as large as 5-10 nm in diameter. As the pH is reduced, there is a tendency for larger silica particles to become even larger and small particles to disappear. Hence, there is a high probability that in addition to silica coated carbon black, there is a large number of free-standing silica particles. Free-standing silica is undesirable because filler-filler interactions in rubber compounds will be high which causes a deterioration of performance of tire-tread compounds.
Another disadvantage of the method described by Kokai No. 63-63755 is that the silica-coated carbon black must be filtered and washed in an attempt to remove dissolved sodium, which is in the form of disodium oxide (Na2O) or sodium hydroxide (NaOH), both of which are corrosive. Owing to its small particle size, carbon black slurries are difficult to filter, and filtration steps to isolate the silica-coated carbon black are time-consuming and costly.
In one embodiment of the present invention, the present invention relates to a method of making a silica-coated carbon product. The method comprises contacting an aqueous media and a carbon product with a solution comprising a silicate substantially free of metal ions, for a sufficient time and temperature to form the silica-coated carbon product. Preferably, the aqueous media is an aqueous slurry containing the carbon product.
In another embodiment of the present invention, the present invention relates to a method of making a silica-coated carbon product by contacting an aqueous media and a carbon product with a solution comprising monosilicic acid, for a sufficient time and temperature to form a silica-coated carbon product.
The present invention also relates to a method of making a silica-coated carbon product, comprising exchanging metal ions in a solution comprising metallic silicate for hydrogen ions and contacting an aqueous media and a carbon product with the solution for a sufficient time and at a sufficient temperature to form a silica-coated carbon product.
Also, the present invention relates to a plurality of silica-coated carbon products, wherein each silica-coated carbon product is substantially and uniformly coated with silica, and the plurality of silica-coated products are in the substantial absence of free silica and/or metal ions.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are intended to provide further explanation of the present invention, as claimed.